Fan control (NSA325)

The longer version:
There is a daemon /usr/sbin/app_wd, which, among other actions, checks the cpu temperature and fan speed every 10 seconds. It does so by reading an i2c device:

For cpu temperature:

i2cget -y 0x0 0x0a 0x07

The output is the temperature in degrees Celcius (hexadecimal)

For fan speed:

i2cget -y 0x0 0x0a 0x08

The output is the time in msec of one rotation (hexadecimal), or in other words, rpm = 60000 / value. Zero means the fan is not running.

By exchanging /sbin/i2cget by a script, I could manipulate the output, and study the behavior when temperatures got high. The fan speed does not change.
When the temperature is too long too high, or the fan is too long too slow, the daemon starts the beeper. At 80 degrees (maybe earlier) to box is beeping. At 112 degrees (maybe earlier) the box is shut down.